Unlike mice and humans, some vertebrate species generate a diverse primary antibody repertoire in gut- associated lymphoid tissues (GALT), where B cells mutate an initial repertoire limited in diversity by preferential use of a small number of V, (D) and J gene segments during immunoglobulin (Ig) gene rearrangement in the bone marrow. Antibody diversification in GALT occurs in an antigen (Ag)- and T cell- independent manner and, in some species (e.g. rabbits), is dependent on select members of the intestinal microbiota. Although humans and mice utilize a different strategy, generating a highly diverse primary antibody repertoire through combinatorial rearrangement of many different V, (D) and J gene segments, we hypothesize that the former strategy is conserved in some human and mouse B cell populations. Casola et al. (2004), for example, found that B cells lacking B cell receptors spontaneously developed germinal centers (GCs) in Peyer's patches. This observation, demonstrating that B cells in mouse GALT can be activated by signals derived from the intestinal microbiota in an Ag-independent manner, is strikingly reminiscent of the Ag-independent B cell activation by intestinal commensals that drives antibody repertoire diversification in rabbit GALT. Similarly, Weller et al. (2004) identified a circulating human B cel population that expresses a somatically hypermutated antibody repertoire, even in patients lacking functional CD40 or CD40L, and hence unable to mount T-dependent immune responses or support affinity maturation. These, among other, lines of evidence lead us to hypothesize that Ag- and T cell-independent antibody repertoire diversification in GALT is conserved in some primate and rodent B cell populations.